DE102013107241A1 - Conducting salt for metal-air batteries - Google Patents

Abstract

The invention relates to a metal-air or metal-oxygen energy store comprising a metal 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate salt, in particular lithium 2-pentafluoroethoxy-1,1,2, 2-tetrafluoro-ethanesulfonate, and their use as conductive salt in metal-air or metal-oxygen energy storage, especially in lithium-air or lithium-oxygen batteries or batteries.

Description

The invention relates to a conductive salt and an electrolyte for metal-air or metal-oxygen batteries.

Recently, research in the field of energy storage and conversion has focused on metal-air or metal-oxygen systems. The principle of operation of the metal-air or oxygen batteries is based on the dissolution of metal ions from a negative metal electrode, followed by their transport through an electrolyte to a positive gas diffusion electrode (GDE), while electrons are transported to the positive electrode via an external electrical circuit. At the positive electrode, the metal ions are not intercalated as in conventional lithium-ion batteries, but react with oxygen. For lithium-air or oxygen batteries, a simplified electrochemical discharge reaction to Li ₂ O ₂ can be described as follows: 2Li ⁺ + O ₂ + 2e ^- → Li ₂ O ₂ .

Such systems offer the advantage over conventional lithium-ion based energy stores and converters that the cathode active material such as oxygen does not have to be stored in the cathode itself, but is supplied from the air, resulting in higher energy densities. For example, lithium-air batteries have more than ten times higher theoretical specific energy than conventional lithium-ion batteries, comparable to conventional fuels. Despite the high theoretical performance of these systems, in practice, lower values are obtained.

This is due in particular to restrictions of the electrolyte in the cell. The limitations include, among other things, the decreased stability of the electrolyte to reduced intermediates formed in the cell during electrochemical reactions with oxygen, as well as stability to the cathode, anode, and other cell components.

Currently lithium hexafluorophosphate (LiPF ₆ ) is used as the conductive salt in lithium-air or oxygen systems. However, lithium hexafluorophosphate has significant disadvantages due to its sensitivity to hydrolysis and lack of stability to O ₂ ^- and Li ₂ O ₂ . Furthermore, LiCF ₃ SO ₃ (Lithiumtrifluoromethansulfonat, LiTf) and LiN (CF ₃ SO ₂ ) ₂ (lithium bis (trifluoromethanesulfonyl) imide, LiTFSI) are used, but favor the dissolution of the commonly used aluminum current collector. This phenomenon is called aluminum corrosion.

In order to exploit the potential of the technology, suitable electrolytes are required. Because it is an open system, the requirements for electrolytes for metal-air or metal-oxygen systems differ from those for lithium-ion systems. The requirements of an electrolyte for metal-air or metal-oxygen systems include not only a wide electrochemical stability window, high ionic conductivity and stability against the metal anode and aluminum current collectors, but in particular also stability to oxygen and reduced forms of oxygen.

The present invention was therefore based on the object to provide a compound which overcomes at least one of the aforementioned disadvantages of the prior art. In particular, the object of the present invention was to provide a compound suitable as conductive salt for metal-air or metal-oxygen batteries.

This object is achieved by a metal-air or metal-oxygen energy storage, comprising a metal 2-pentafluoroethoxy-1,1,2,2-tetrafluoro-ethanesulfonate salt.

Further objects of the invention relate to an electrolyte for a metal-air or metal-oxygen energy storage comprising a metal 2-pentafluoroethoxy-1,1,2,2-tetrafluoro-ethanesulfonate salt as the electrolyte salt and a solvent, and the Use of a metal 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate salt in metal-air or metal-oxygen energy stores.

Surprisingly, it has been found that metal 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate salts are useful in metal-air or metal-oxygen systems. The metal salts based on 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate anions (FEES ^- anions) show a high ionic conductivity and a wide electrochemical stability window. It is furthermore advantageous that the metal salts based on 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate anions do not promote the dissolution of aluminum, which is usually used as a current collector. In addition, another advantage of the metal salts based on 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate is a low sensitivity to hydrolysis. Of particular advantage, however, is a surprisingly found high resistance to reduced forms of oxygen such as O ₂ ^- or Superoxide and a high stability in the presence of metal oxides.

The terms "metal-air energy storage" and "metal-oxygen energy storage" in the context of the present invention include systems based on a reaction with oxygen. Metal-air and metal-oxygen systems include a negative metal electrode and a positive gas diffusion electrode (GDE), often referred to as an "air cathode". The electrochemical reaction at the cathode comprises a reaction with oxygen. This is usually supplied via the ambient air or can be supplied in the form of oxygen gas.

The term "energy storage" in the context of the present invention comprises primary and secondary electrochemical energy storage device, ie batteries (primary storage) and accumulators (secondary storage). However, in common usage, accumulators are often also referred to by the term battery often used as a generic term. Thus, the term lithium-air battery is used synonymously with lithium-air battery. In this respect, the term "lithium-air battery" in the present case also refer to a "lithium-air battery". In addition, the concept of the lithium-air battery and the lithium-air battery is similar or identical to the concept of the lithium-oxygen battery and the lithium-oxygen battery. Therefore, these systems are generally understood by the term "metal-air battery".

In preferred embodiments, the energy store is selected from the group comprising lithium, sodium, magnesium, aluminum, zinc and / or iron-air or oxygen storage batteries or batteries. Compared to the classic lead-acid, nickel-cadmium and lithium-ion systems, air systems such as zinc-air show an improvement in specific energy. In addition to zinc, in particular lithium or sodium air or oxygen storage batteries and batteries are of commercial interest.

In a particularly preferred embodiment, the energy store is a lithium-air or lithium-oxygen battery or a lithium-air or lithium-oxygen battery. Advantageously, the performance of the lithium-air system is comparable to that of gasoline.

In electrochemical energy storage such as metal-air batteries, charge transport by an electrolyte is concerned. A liquid electrolyte contains a conductive salt dissolved in a solvent. Electrochemical energy stores therefore contain an electrolyte in which the metal is contained in ionic form.

With respect to metal-air batteries in which the anode or the negative electrode material of lithium, sodium, magnesium, aluminum, zinc and iron metal is formed, the salt preferably comprises the corresponding metal ions or Salts based on 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate or FEES ^- anions. The metal 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate salts are therefore preferably selected from the group comprising lithium, sodium, magnesium, aluminum, zinc and / or iron 2-pentafluoroethoxy -1,1,2,2-tetrafluoro-ethane sulfonate.

In a preferred embodiment, the salt is lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate. Lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate is also referred to as lithium 1,1,2,2-tetrafluoro-2- (pentafluoroethoxy) ethanesulfonate. Lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate (LiFEES) shows a particularly good resistance to reduced forms of oxygen such as O ₂ ^- or superoxides and a high stability in the presence of Li ₂ O ₂ and Li ₂ Furthermore, lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate has a high ionic conductivity and a wide electrochemical stability window. Furthermore, lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate salts do not promote the dissolution of aluminum, providing a major advantage over the LiTf and LiTFSI salts useful in lithium-air systems. Compared to LiPF ₆ , the advantages are lower hydrolysis sensitivity and higher thermal stability.

Preference is given to organic electrolytes and electrolytes based on ionic liquids. In preferred embodiments, the metal salt, in particular lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate, in an electrolyte comprising a solvent selected from the group comprising glycol dialkyl ethers and their oligomers and polymers, amides, sulfoxides, nitriles, Phosphonates, phosphate ethers, fluorinated ethers, substituted 2-oxazolidinones, ionic liquids and / or mixtures thereof.

Preferred amides and lactams are selected from the group comprising dimethylformamide (DMF), dimethylacetamide (DMA) and / or N-methyl-2-pyrrolidone (NMP). A preferred sulfoxide is dimethylsulfoxide (DMSO). Preferred nitriles are selected from the group comprising acetonitrile (AN), 1,4-dicyanobutane (adiponitrile, AND) and / or aromatic nitriles such as 2-tolunitrile and benzonitrile (BN). A preferred phosphonate or phosphate ether is tris (2,2,2-trifluoroethyl) phosphate (TFP). Preferred solvents are selected from the group comprising dimethylformamide (DMF), dimethylacetamide (DMA), N-methyl-2-pyrrolidone (NMP), 2-tolunitrile, benzonitrile, tris (2,2,2-trifluoroethyl) phosphate (TFP), Dimethyl sulfoxide (DMSO) and / or mixtures thereof.

Particularly preferred solvents are glycol dialkyl ethers (glymes) of the general formula R ¹ -O- (CH ₂ CH ₂ -O) _n -R ² where R ¹ and R ² are each independently CH ₃ , C ₂ H ₅ , C ₃ H ₇ or C ₄ H ₉ and n = 1 to 50 and mixtures thereof. Glycol dialkyl ethers are chemically and thermally stable. Thus, glycol dialkyl ethers withstand temperatures of over 200 ° C without decomposing. Preferred glycol dialkyl ethers are selected from the group comprising 1,2-dimethoxyethane (ethylene glycol dimethyl ether or monoglyme), bis (2-methoxyethyl) ether (diglyme), triethylene glycol dimethyl ether (triglyme) and / or tetraethylene glycol dimethyl ether (tetraglyme).

Further preferred are glycol dialkyl ethers of the general formula R ¹ -O- (CH ₂ CH ₂ -O) _n -R ² where R ¹ and R ² are each independently CH ₃ , C ₂ H ₅ , C ₃ H ₇ or C ₄ H ₉ and n = 2 to 50. Particularly preferred glycol dialkyl ethers are selected from the group comprising bis (2-methoxyethyl) ether (diglyme), triethylene glycol dimethyl ether (triglyme) and / or tetraethylene glycol dimethyl ether (tetraglyme). These have a higher boiling point and are advantageously used in an open battery system. Very particularly preferred is tetraethylene glycol dimethyl ether (TEG-DME), which is also referred to as 2,5,8,11,14-Pentaoxapentadecan.

Metal-air and metal-oxygen batteries comprising an electrolyte of metal 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate salt dissolved in tetraethylene glycol dimethyl ether (TEG-DME), especially lithium 2-pentafluoroethoxy -1,1,2,2-tetrafluoroethanesulfonate included are advantageously particularly stable to reduced forms of oxygen such as O ₂ ^- or superoxides. The high stability of the electrolyte system of lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate (LiFEES) in tetraethylene glycol dimethyl ether to reduced forms of oxygen was experimentally demonstrated by the presence of Li ₂ O ₂ as the desired product of the electrode reaction to be shown.

The formation of Li ₂ O ₂ in the electrode reaction is also dependent on the electrolyte, in particular the solvent and salt. Thus, no Li ₂ O _{2 is formed} in the carbonate solvents such as ethylene carbonate used in lithium-ion batteries because the high reactivity of the carbonates and reduced forms of oxygen prevent this. Therefore, cyclic or linear carbonates are preferably not usable as solvents in metal-air and metal-oxygen batteries, and electrolytes for metal-air and metal-oxygen batteries preferably do not contain cyclic or linear carbonates as solvents.

Also preferred solvents for metal-air and metal-oxygen batteries are ionic liquids which combine high thermal and electrochemical stability with high ionic conductivity. Preferred ionic liquids include a cation selected from the group consisting of 1-ethyl-3-methylimidazolium (EMI ⁺ ), 1,2-dimethyl-3-propylimidazolium (DMPI ⁺ ), 1,2-diethyl-3,5-dimethylimidazolium (DEDMI ⁺ ), Trimethyl-n-hexylammonium (TMHA ⁺ ), N-alkyl-N-methylpyrrolidinium (PYR _1R ⁺ ), N-alkyl-N-methylpiperidinium (PIP _1R ⁺ ) and / or N-alkyl-N-methylmorpholinium (MORP _1R ⁺ ) and an anion selected from the group comprising bis (trifluoromethanesulfonyl) imide (TFSI ^- ), bis (pentafluoroethanesulfonyl) imide (BETA), bis (fluorosulfonyl) imide (FSI ^- ), 2,2,2-trifluoro-N- (trifluoromethanesulfonyl) acetamide (TSAC ^- ), tetrafluoroborate (BF ₄ ^- ), pentafluoroethane trifluoroborates (C ₂ F ₅ BF ₃ ^- ), hexafluorophosphate (PF ₆ ^- ) and / or tri (pentafluoroethane) trifluorophosphate ((C ₂ F ₅ ) ₃ PF ₃ ^- ). Preferred N-alkyl-N-methylpyrrolidinium (PYR _1R ⁺ ) cations are selected from the group comprising N-butyl-N-methylpyrrolidinium (PYR ₁₄ ⁺ ) and / or N-methyl-N-propylpyrrolidinium (PYR ₁₃ ⁺ ). Preferred ionic liquids are selected from the group comprising N-butyl-N-methylpyrrolidinium bis (trifluoromethanesulfonyl) imide (PYR ₁₄ TFSI) and / or N-methyl-N-propylpyrrolidinium bis (trifluoromethanesulfonyl) imide (PYR ₁₃ TFSI).

The concentration of the metal 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate salt, in particular lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate, in the electrolyte can be in the range of ≥ 0 , 2 M to ≦ 1.6 M, preferably in the range from ≥ 0.6 M to ≦ 1.4 M, preferably in the range from ≥ 0.8 M to ≦ 1.2 M. Particular preference may be given to the metal salts, in particular lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate, in a concentration of 1 M.

Another object of the invention relates to an electrolyte for a metal-air or metal-oxygen energy storage comprising a metal 2-pentafluoroethoxy-1,1,2,2-tetrafluoro-ethanesulfonate salt, in particular lithium 2-pentafluoroethoxy-1 , 1,2,2-tetrafluoroethanesulfonate, as an electrolyte salt and a solvent selected from the group comprising glycol dialkyl ethers of the general formula R ¹ -O- (CH ₂ CH ₂ -O) _n -R ² where R ¹ and R ² are independently CH ₃ , C ₂ H ₅ , C ₃ H ₇ or C ₄ H ₉ and n = 2 to 50, dimethylformamide (DMF), dimethylacetamide (DMA), N-methyl-2-pyrrolidone (NMP), 2- Tolunitrile, benzonitrile, tris (2,2,2- trifluoroethyl) phosphate (TFP), dimethyl sulfoxide (DMSO) and / or mixtures thereof.

The metal 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate salts are preferably selected from the group comprising lithium, sodium, magnesium, aluminum, zinc and / or iron 2-pentafluoroethoxy 1,1,2,2-tetra-fluoro-ethane sulfonate. Preferably, the salt is lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate.

A preferred solvent is dimethyl sulfoxide (DMSO). Particularly preferred solvents are glycol dialkyl ethers of the general formula R ¹ -O- (CH ₂ CH ₂ -O) _n -R ² , where R ¹ and R ² are each independently CH ₃ , C ₂ H ₅ , C ₃ H ₇ or C ₄ H ₉ and n = 2 to 50, and mixtures thereof. Preferred glycol dialkyl ethers (glymes) are selected from the group comprising bis (2-methoxyethyl) ether (diglyme), triethylene glycol dimethyl ether (triglyme) and / or tetraethylene glycol dimethyl ether (tetraglyme). These have a higher boiling point and are advantageously used in an open battery system. In preferred embodiments, the solvent is tetraethylene glycol dimethyl ether. Electrolytes containing the metal 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate salt dissolved in tetraethylene glycol dimethyl ether (TEG-DME), in particular lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate, are advantageously particularly stable to reduced forms of oxygen such as O ₂ ^- or superoxides. These are therefore particularly well suited for metal-air and metal-oxygen batteries. Electrolytes for metal-air and metal-oxygen batteries preferably contain no cyclic or linear carbonates as solvents as used in lithium-ion batteries.

In preferred embodiments, the concentration of the metal salt, in particular lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate, in the electrolyte is in the range of ≥ 0.2 M to ≦ 1.6 M, preferably in the range of ≥ 0.6 M to ≦ 1.4 M, particularly preferably in the range from ≥ 0.8 M to ≦ 1.2 M. In a particularly preferred embodiment, the concentration of metal salt, in particular lithium 2-pentafluoroethoxy-1,1, 2,2-tetrafluoroethanesulfonate, in the electrolyte at 1 M. Advantageously, such concentrations of lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate lead to a good conductivity of the electrolyte.

The metal salts based on 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate anions (FEES ^- anions) are useful as a conductive or electrolyte salt in electrolytes for metal-air or metal-oxygen energy storage. Another object of the invention relates to the use of a metal 2-pentafluoroethoxy-1,1,2,2-tetrafluoro-ethanesulfonate salt in metal-air or metal-oxygen energy storage, especially in lithium-air or lithium-oxygen Accumulators or batteries. Preference is given to the use of lithium, sodium, magnesium, aluminum, zinc and / or iron 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate, in particular of lithium 2-pentafluoroethoxy-1, 1,2,2-tetrafluoro-ethane sulfonate.

In an advantageous manner, in particular by using lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate in lithium-air or lithium-oxygen storage batteries or batteries, a conductive salt can be provided which is thermal and opposite moisture is stable and resistant to hydrolysis, does not support resolution of an aluminum current collector and the stable to O ₂ ^- or superoxides.

The metal 2-pentafluoroethoxy-1,1,2,2-tetrafluoro-ethanesulfonate salts can be prepared by customary synthesis methods, for example by reacting perfluoro (2-ethoxyethane) sulfonic acid with the corresponding metal hydroxide. For example, lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate is also commercially available.

In a preferred embodiment, the metal salt, in particular lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate, in an electrolyte comprising a solvent selected from the group comprising glycol dialkyl ethers of the general formula R ¹ -O- (CH ₂ CH ₂ -O) _n -R ² where R ¹ and R ² are each independently CH ₃ , C ₂ H ₅ , C ₃ H ₇ or C ₄ H ₉ , and n = 2 to 50, dimethylformamide, dimethylacetamide, N-methyl 2-pyrrolidone, 2-tolunitrile, benzonitrile, tris (2,2,2-trifluoroethyl) phosphate, dimethyl sulfoxide and / or mixtures thereof.

Preferred glycol dialkyl ethers (glymes) are selected from the group comprising bis (2-methoxyethyl) ether (diglyme), triethylene glycol dimethyl ether (triglyme) and / or tetraethylene glycol dimethyl ether (tetraglyme). Particularly preferred embodiments is the metal salt, in particular lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate, used in tetraethylene glycol dimethyl ether as a solvent. Electrolytes containing the metal 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate salt dissolved in tetraethylene glycol dimethyl ether (TEG-DME), in particular lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate, are particularly useful for metal-air and metal-oxygen batteries because they are particularly stable to reduced forms of oxygen such as O ₂ ^- or superoxides.

Examples and figures which serve to illustrate the present invention are given below.

Here are the figures:

1 shows a schematic view of the lithium-air / oxygen cell used.

2 shows the electrochemical stability window of electrolytes containing in each case 1 M LiSO ₃ C ₂ F ₄ OC ₂ F ₅ (LiFEES), 1 M LiPF ₆ or 1 M LiSO ₃ CF ₃ (LiTf) in TEG-DME, measured on gold electrodes at one Potential feed rate of 0.075 mV / s between the initial potential and the end potential of 5.5 V versus Li / Li ⁺ .

3 shows a comparison of electrolytes containing each 1 M LiPF ₆ and 1 M LiFEES (LiSO ₃ C ₂ F ₄ OC ₂ F ₅ ) in TEG-DME by linear sweep voltammetry, measured with Al disks as electrodes at a potential feed rate of 0.075 mV / s between the initial potential and the end potential of 5.5 V versus Li / Li ⁺ .

4 Figure 12 shows a comparison of the capacities in the first discharge cycle of uncatalyzed and catalyzed gas diffusion electrodes using an electrolyte solution of 1 M LiFEES in TEG-DME measured at a current density of 5 mA / g of carbon in dry air.

5 Figure 10 shows a comparison of the capacities during the first cycle of carbon-gas diffusion electrodes (VXC72R) in oxygen using an electrolyte solution of 1 M LiFEES in TEG-DME measured at a current density of 10 mA / g of carbon.

6 Figure 12 shows a comparison of the X-ray diffractograms of pure Li ₂ O ₂ (bottom) of an unused carbon gas diffusion electrode (center) and an identical gas diffusion electrode after discharge to 2 V versus Li / Li ⁺ in oxygen atmosphere (above) using an electrolyte solution of 1 M LiFEES in TEG-DME. 6a ) shows the X-ray diffractogram between 20 ° and 43 ° 2θ degrees, 6b ) X-ray diffractogram between 55 ° and 62 ° 2θ degrees.

7 Figure 12 shows a comparison of the electrochemical ac impedance measurements of a lithium / carbon gas diffusion electrode cell using an electrolyte solution of (a) 1 M LiPF ₆ and (b) 1 M LiFEES in TEG-DME.

8th FIG. 12 shows a comparison of the AC-impedance electrochemical impedance measurements of a carbon-gas diffusion-electrode / carbon-gas diffusion-cell symmetrical cell using an electrolyte solution of (a) 1 M LiPF ₆ and (b) 1 M LiFEES in TEG-DME.

9 FIG. 12 shows a comparison of the AC electrochemical impedance measurements of a balanced cell with Li / Li electrodes using an electrolyte solution of (a) 1 M LiPF ₆ and (b) 1 M LiFEES in TEG-DME.

example 1

Preparation of lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate (LiFEES)

15.0 g (47.5 mmol) of perfluoro (2-ethoxyethane) sulfonic acid (ABCR, 95%) were mixed with 2.01 g (47.9 mmol) of lithium hydroxide monohydrate (Sigma-Aldrich, puriss. Pa, ≥ 99.0%). mixed in 50 ml Milli-Q water (Millipore) and stirred for 2 hours at 50 ° C. After removal of the water and recrystallization from an acetonitrile / dichloromethane mixture, lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate was obtained as colorless crystals. By renewed dissolution and removal of the solvent, the salt was present as a white powder, which was dried for 24 hours under vacuum at 80 ° C before use.

Electrochemical Methods and Techniques:

Production of the electrolytes

The preparation of the electrolytes was carried out under a protective gas atmosphere in an argon-filled glove box, with a water and oxygen content of less than 0.1 ppm. The solvent tetraethylene glycol dimethyl ether (TEGDME, Sigma Aldrich, 99%, anhydrous) was first mixed with molecular sieve (4 Å), otherwise used without further workup. Tetraethylene glycol dimethyl ether and lithium tetrafluoro (pentafluoroethoxyethane) sulfonate (LiFEES), which was prepared according to Example 1, were mixed in a weight ratio of 4: 1 (1 molar) for the batch of the electrolyte according to the invention. For the preparation of the comparative electrolyte LiPF ₆ (Sigma Aldrich, 99.99%) and lithium trifluoromethylsulfonate (LiSO ₃ CF ₃ , 3 M, 99.99%) were mixed in each case in the appropriate weight ratio, so that in each case also gave a 1 M solution.

Production of the gas diffusion electrodes

The gas diffusion electrodes were after the Method of Read et al., The Journal of Electrochemical Society, Vol. 149, page A1190 and Vol. 150, page A1351 , manufactured. To a dispersion of Vulcan carbon (VXC72R, Cabot) in isopropanol was added an aqueous suspension containing 60% by weight of PTFE (polytetrafluoroethylene, Sigma Aldrich). given. The slurry obtained was applied by film casting with a layer thickness of 100 microns on an aluminum wire mesh (53 microns mesh, Paul GmbH & Co). To remove residual isopropanol and water, the gas electrode was pre-dried at 80 ° C. After the first pre-drying, the gas electrodes were pressed through a roller press with 150 μm gap at 60 ° C (Heat Roll Press HSAM-615H, Hosen Corp., Japan). From the calendared electrodes, disks with a diameter of 10 mm were punched and further dried for 12 hours at 150 ° C. under reduced pressure. The composition of the electrode by weight ratio was Vulcan carbon (C): PTFE = 80:20.

Gas-diffusion electrodes with electrocatalyst based on α-MnO ₂ and acid-treated Li ₂ MnO ₃ (AT-LMO) were prepared according to the corresponding manufacturing process.

Gas diffusion electrode with electrocatalyst based on α-MnO ₂ :
First, nanowires of α-MnO _{2 were} synthesized by dissolving 1 mmol each of KMnO ₄ (ABCR, 98%) and NH ₄ Cl (ABCR, 98%) in 50 ml of water and placing them in an autoclave so that it was 80% was filled to volume. The autoclave was heated at 140 ° C for 24 hours in the oven (binder). At the end of the time, the autoclave was allowed to cool to room temperature. After filtration of the contents, a powder was obtained. The filter residue was washed several times with demineralised water before a final wash with ethanol. The powder was dried at 60 ° C for one hour in vacuo and then overnight at 100 ° C. The preparation of the gas diffusion electrode then takes place as indicated above. The composition of the electrode by weight ratio was C: PTFE: α-MnO ₂ = 70:15:15.

Gas diffusion electrode with electrocatalyst based on acid-treated Li ₂ MnO ₃ :
The synthesis of Li ₂ MnO ₃ initially involved dissolving a stoichiometric amount of the powdered precursors of manganese (II) acetate tetrahydrate (ABCR, 99%) and lithium acetate monohydrate (ABCR, 99%) in a mixture of ethylene glycol (BDH Prolabo 99.7 %) and citric acid (Grüssing GmbH 99%). The molar ratio of ethylene glycol (EG) to citric acid (CA) was 4: 1 and the molar ratio of ethylene glycol (EG) to manganese (II) ions was 10: 1. Complete dissolution of the reagents was then carried out by heating with stirring to 90 ° C. Subsequently, the temperature was raised to 140 ° C to initiate the esterification reaction. Thereafter, the temperature was raised to 180 ° C for 12 hours to form the polyester. The following was heated to 250 ° C for 6 hours, whereby the ethylene glycol volatilized and the sample caramelized appeared. The sample was then calcined at 450 ° C for 5 hours in a high temperature oven (Nabertherm), whereby the precursors decompose. Subsequent acid treatment involved stirring the resulting powder in an 8M solution of H ₂ SO ₄ for about one week at 50 ° C. After one week, the powder was filtered, washed several times with demineralised water until the pH was neutral and predried at 80 ° C for 12 hours before finally drying at 300 ° C for a further 6 hours. The preparation of the gas diffusion electrode then takes place as indicated above. The composition of the electrode by weight ratio was C: PTFE: AT-LMO = 67:15:18. the figure refers to the acid-treated form of the Li ₂ MnO ₃ material (AT-LMO).

Production of electrochemical metal-air / oxygen cell

The cells for the electrochemical measurements were fabricated under a protective gas atmosphere in an argon-filled glove box, with a water and oxygen content of less than 0.1 ppm.

A schematic view of the cell for the electrochemical measurements is in 1 shown. The construction of the metal-air / oxygen cell 1 based on a Swagelok ^® cell 2 , as used for electrochemical measurements of lithium-ion systems, wherein a modified component was provided for the working electrode to a gas diffusion electrode 3 (GDE) with gas exchange for air or oxygen. This is made possible by an open cathode side. The gas reservoir 5 allows the gas supply only to the back of the positive gas diffusion electrode 3 (GDE), causing unwanted side reactions of the gases with the metal anode 4 (GE) and the reference electrode 8th be prevented. The cell also had a stainless steel sieve 10 (Frit) and a steel spring 11 that contact a stainless steel rod 12 has, the channels 6 and 7 for gas supply and gas outlet contains. These gas holes allow the cell to be flushed with the desired gas. Separated was the gas diffusion electrode 3 (GDE) from the counter electrodes through a glass fiber separator 9 ,

Lithium-metal foil (Rockwood Lithium) was used both as a counter electrode (GE) and as a reference electrode (RE). Separately, the gas diffusion electrodes (GDE) were separated from the counter electrodes by glass fiber separators (Whatman, GF / D) soaked in the respective electrolyte.

In the experiments, a negative electrode of metallic lithium was used. The These potentials thus refer to the redox couple Li / Li ⁺ . The electrochemical tests were carried out at a temperature of 20 ° C ± 2 ° C. As potentiostat / galvanostat a BaSyTec GSM (BaSyTec GmbH) battery test system was used.

Example 2

Determination of the electrochemical stability window

The electrochemical stability window of an electrolyte comprising 1 M LiSO ₃ C ₂ F ₄ OC ₂ F ₅ (lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate, LiFEES) in tetraethylene glycol dimethyl ether and comparative electrolytes was investigated 1 M LiPF ₆ or 1 M LiSO ₃ CF ₃ (lithium trifluoromethylsulfonate, LiTf) in tetraethylene glycol dimethyl ether (TEG-DME).

The electrochemical stability window of the electrolyte was determined in each case in a conventional Swagelok ^® cell on a BioLogic VSP Electrochemical Workstation (BioLogic) using linear sweep voltammetry (LSV). The working electrode used was a gold microelectrode (Metrohm Autolab BV, Au 3 mm) with a geometric surface area of 0.0707 cm ² , while metallic lithium was used both as counter electrode and as reference electrode. At a rate of 0.075 mV / s, the potential of the working electrode between the initial potential (OCV), the no-load voltage at which no current flows, and the end potential of 5.5V. Li / Li ⁺ scanned.

The 2 shows the electrochemical stability window of the electrolyte containing in each case 1 M LiSO ₃ C ₂ F ₄ OC ₂ F ₅ (lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate, LiFEES), 1 M LiPF ₆ or 1 M LiSO ₃ CF ₃ (lithium fluoromethanesulfonate, LiTf) in tetraethylene glycol dimethyl ether. Again 2 can be taken, corresponds to the electrochemical stability of the electrolyte containing 1 M lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoro-ethanesulfonate the stability of the electrolyte containing lithium hexafluorophosphate (LiPF ₆ ) or Lithiumfluoromethansulfonat.

This stability allows lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate such as lithium hexafluorophosphate or lithium fluoromethanesulfonate to be used as the conductive salt in lithium-air or lithium-oxygen cells. However, lithium fluoromethanesulfonate is known to aid in the dissolution of aluminum.

Example 3

Investigation on aluminum electrodes

The dissolution of aluminum was investigated by TEG-DME-based electrolytes containing 1 M LiPF ₆ or lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate (LiFEES, LiSO ₃ C ₂ F ₄ OC ₂ F ₅ ) as the lithium ion-conducting salt in tetraethylene glycol dimethyl ether.

The experiments were carried out in each case in a conventional Swagelok ^® cell on a BioLogic VSP electrochemical workstation (BioLogic) using linear sweep voltammetry (LSV). The working electrode used was a 12 mm diameter aluminum disc having a geometric surface area of 1.1131 cm ² , while metallic lithium was used both as a counter electrode and as a reference electrode. At a rate of 0.075 mV / s, the potential of the working electrode between the initial potential (OCV), the no-load voltage at which no current flows, and the end potential of 5.5V. Li / Li ⁺ scanned.

The 3 shows the comparison of the electrolytes containing in each case 1 M LiPF ₆ and LiSO ₃ C ₂ F ₄ OC ₂ F ₅ in TEG-DME measured with Al disks as electrodes up to a final potential of 5.5 V against Li / Li ⁺ . Again 3 The electrolyte containing LiFEES as a lithium ion-conducting salt in TEG-DME as a solvent, as well as the electrolyte based on LiPF ₆ , showed no significant current. This shows that LiFEES salts such as LiPF ₆ do not promote the dissolution of aluminum. Compared to LiPF ₆ , the advantage of LiFEES is the higher resistance to moisture.

Example 4

Cyclization of a lithium-air cell with constant current

Cyclization with constant current was performed on a BaSyTec GSM battery test system (BaSyTec GmbH). The Zyklisierungsexperimente were carried out in dry air at atmospheric pressure and 20 ° C in an above-described modified for gas diffusion electrodes Swagelok ^® cell. Dry air served as a source of oxygen. The lithium-air cells were in a potential range between 2 V and 4.5 V vs. Li / Li ⁺ at a current density of 5 mA / g, based on the amount of carbon-active material in the electrode, charged and charged. As the gas diffusion electrodes, a carbon gas diffusion electrode prepared as described above, and gas diffusion electrodes with Electrocatalyst based on α-MnO ₂ and acid-treated Li ₂ MnO ₃ used. The electrolyte used was a 1 M solution of lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate (LiFEES) in tetraethylene glycol dimethyl ether (TEG-DME).

The 4 shows the voltage profile of the lithium-air cells, which are up to 2 V vs. Li / Li ^{+ were} discharged in dry air using 1 M LiFEES (lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoro-ethane sulfonate) in TEG-DME as the electrolyte. The specific capacity obtained when discharging correlates with the amount of Li ₂ O ₂ formed in the electrode as a result of the oxygen reduction reaction. The specific discharge capacities obtained in the electrochemical reaction ranged from 405 mAh / g for the pure carbon electrode (no catalyst) above 776 mAh / g for the carbon electrode with α-MnO ₂ as the catalyst (α-MnO ₂ ) to 1192 mAh / g for the acid-treated as a catalyst Li ₂ MnO ₃ -containing electrode (AT-LMO). In addition, the catalyst also affected the potential plateau where the discharge reaction occurred. As a result, the potential plateau indicating the thermodynamic potential of Li ₂ O ₂ (ie, 2.96 V vs. Li / Li ⁺ ) was 0.36 V for pure carbon electrodes and 0.38 V for α MnO ₂ -containing, respectively Electrodes and at 0.3 V for the Li ₂ MnO ₃ electrode.

These results show that the salt lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoro-ethane sulfonate has remarkably high electrochemical performance in all electrodes. In addition, the addition of the catalyst to the electrode composition led to an increase in the specific discharge capacity and a lowering of the overvoltage during discharge.

Example 5

Cyclization of a lithium-oxygen cell with constant current

Cyclization with constant current strength were performed on a GSM BaSyTec battery test system (BaSyTec GmbH) at atmospheric pressure and 20 ° C in an above-described modified for gas diffusion electrodes Swagelok ^® cell performed. Oxygen gas (Westfalen AG, 5.5) served as an oxygen source. The lithium-oxygen cell was exposed to a potential of 2 V at a current density of 10 mA / g, based on the amount of carbon-active material in the electrode. Discharge Li / Li ⁺ . As the gas diffusion electrode, a carbon gas diffusion electrode prepared as described above was used. The electrolyte used was a 1 M solution of lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate (LiFEES) in tetraethylene glycol dimethyl ether (TEG-DME).

5 shows the voltage profile of pure carbon electrodes (VXC72R), which is up to 2 V vs. Li / Li ⁺ were discharged in lithium-air cells, using 1 M LiFEES (lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoro-ethane sulfonate) in TEG-DME as the electrolyte. In contrast to Example 4 was discharged in pure oxygen atmosphere. Again 5 The carbon electrode was capable of carrying a specific capacity of 561 mAh in an electrolyte containing 1 M LiFEES (lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate) and by supplying oxygen and the potential plateau, which indicated the thermodynamic potential of Li ₂ O ₂ (ie 2.96 V vs. Li / Li ⁺ ), dropped to 0.26 V.

The resulting discharge capacity as well as the potential plateau of the electrochemical discharge reaction showed an expected better behavior than the use of air. Thus, the salt lithium-2-pentafluoroethoxy-1,1,2,2-tetrafluoro-ethane-sulfonate also showed a remarkable electrochemical performance even when using pure oxygen in TEG-DME.

Example 6

X-ray diffraction

The discharged carbon gas diffusion electrode of Example 5 was characterized by X-ray diffractometry with respect to its phase composition. The diffraction measurements were performed on an AXS Bruker D8 Advance diffractometer operating in the θ-θ mode and equipped with a copper anti-cathode (λCu = 1.54 Å), bichromatic Kα1 and Kα2, and LynxEye 1-dimensional PSD detector , The data acquisition was carried out in the 2θ degree range from 20 ° to 43 ° and from 55 ° to 62 °, at a time / step ratio of 2 s and an increment change of 0.02 ° / step.

For the measurement, the discharged electrode was removed from the cell in a glove box under an argon atmosphere, applied directly to a sample holder for X-ray diffraction (silicon single crystal) and sealed using special Scotch Magic Tape (3 M) , The samples were processed in a glove box under an argon atmosphere so that the Li ₂ O ₂ formed by the discharge reaction would not hydrolyze to LiOH.

The 6 ₃ shows a comparison of the X-ray diffractograms of the pure Li ₂ O ₂ (below) of the unused carbon gas diffusion electrode (center) and the carbon gas used in Example 5. Diffusion electrode after discharge to 2 V vs. Li / Li ⁺ in oxygen atmosphere (top). 6a ) shows the X-ray diffractogram between 20 ° and 43 ° 2θ degrees, 6b ) X-ray diffractogram between 55 ° and 62 ° 2θ degrees.

The discharged electrode signals at 33.08 ° (100), 35.25 ° (101) and 58.91 ° (110) indicate that Li ₂ O _{2 was formed} as a discharge product. The signal at 38.32 ° corresponds to the signal of the aluminum wire mesh of the carbon-gas diffusion electrode. The presence of Li ₂ O ₂ as the desired discharge product demonstrates the stability of the electrolyte system of the salt LiFEES and the solvent TEG-DME towards reduced forms of oxygen.

These results show that lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate is useful as a conductive salt in lithium-air and lithium-oxygen cells, and particularly in tetraethylene glycol dimethyl ether (TEG-DME) as Solvent can provide a stable electrolyte, which has both stable to reduced forms of oxygen as well as a wide electrochemical stability window and does not promote the aluminum dissolution.

Example 7

The temporal evolution of the interface between electrolyte and lithium metal or gas diffusion electrode was investigated by means of electrochemical impedance spectroscopy (EIS) under OCV conditions at open circuit voltage at which no current flows.

Solutions of 1 M lithium hexafluorophosphate or 1 M lithium 2-pentafluoroethoxy-1,1,2,2 were used as electrolytes for the production of the lithium-air cells, symmetrical cells with gas diffusion electrodes and symmetrical cells with lithium metal Tetrafluoro-ethane sulfonate in tetraethylene glycol dimethyl ether (TEG-DME) used. The AC impedance measurements were performed in the frequency range between 1 MHz and 10 MHz at a voltage amplitude of 5 mV on a Solartron Electrochemical Interface 1287/1260 (Solartron Analytical). The AC impedance measurements took place at 20 ° C and OCV potential.

The lithium-air cells were treated with metallic lithium as the counter electrode and, separated by the electrolyte, the carbon-gas diffusion electrode as a working electrode ( , b) assembled. In the symmetric cells with gas diffusion electrodes, both the counter electrode and the working electrode were made of carbon-gas diffusion electrodes separated from each other by the electrolyte ( , b) used. In the lithium-metal symmetrical cells, both the counter electrode and the working electrode were made of metallic lithium separated by the electrolyte ( , b) used.

Considering the sequence of repeated measurements of the impedance behavior of lithium-air cells as a function of time, the thickening of the electrode / electrolyte interface can be examined. The 7a) and 7b) FIG. 12 shows a comparison of the AC electrochemical impedance measurements using an electrolyte solution of 1 M LiPF ₆ (a) and 1 M LiFEES in TEG-DME (b) of a lithium / carbon gas diffusion electrode cell. In 7a) and 7b) For example, the intersection of the semicircle with the real axis (Z ') shows an increase in resistance related to both the electrolyte and the charge transfer resistance at the electrode-electrolyte interface. For the electrolytes 1 M LiPF ₆ and 1 M LiFEES in TEG-DME, the resistance, ie the point of intersection with the real axis at high frequencies, did not change over time. The increase in the value for the resistance at low frequencies is thus associated with an enlargement of the surface layer at the electrode / electrolyte interface.

The 7a) and 7b) shows that the amplitude of this increase for the 1 M LiPF ₆ electrolyte is greater than when using 1 M LiFEES. This shows that the cell containing the LiPF ₆ -based electrolyte had a higher resistance compared to the cell containing the LiFEES-based electrolyte. Thus, the lithium-air cell according to the invention comprising lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate as conductive salt shows a lower cell resistance and thus a better performance than the lithium-air cell with LiPF ₆ as conductive salt.

The examination of the interface over time was also conducted with respect to the total resistance of a symmetrical cell fabricated with carbon-gas diffusion electrodes. The 8a) and 8b) FIG. 12 shows a comparison of the electrochemical AC impedance measurements using an electrolyte solution of 1 M LiPF ₆ (a) and 1 M LiFEES in TEG-DME (b) of a carbon-gas diffusion-electrode / carbon-gas cell diffusion electrode. How to get the can be found for any of the two electrodes, a change in the resistance between the interfaces in the time course. This shows that the change in the impedance spectrum was not caused by the gas diffusion electrode. In addition, the show Results in 8a) and 8b) are shown that the interface of the carbon-gas diffusion electrodes was stable to both electrolytes.

Over time, EIS investigations of the interface with respect to the total resistance of a symmetrical cell fabricated with lithium-metal electrodes are disclosed in U.S. Patent Nos. 5,396,074; 9a) and 9b) shown. In contrast to 8a) and 8b) the increase in the amplitude of the semicircle indicates an increase in the lithium / electrolyte interface. In both cases, the amplitude of the semicircle reached a constant value, indicating that the enlargement of the SEI (solid-electrolyte interphase) layer ends after some time.

The increase in amplitude was higher for the use of 1 M LiPF ₆ as the electrolyte than for the use of 1 M LiFEES in TEG-DME. This again shows that the cell containing the LiFEES-based electrolyte had a significantly lower resistance compared to the cell containing a LiPF ₆ -based electrolyte. Thus, even with the use of lithium-metal electrodes, the lithium-air cell according to the invention comprising lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate as conductive salt shows a better performance than the lithium-air cell with LiPF ₆ as conductive salt.

Comparing the Nyquist plots of the symmetrical cells made with lithium metal and 1 M LiPF ₆ or 1 M LiFEES in TEG-DME as the electrolyte, qualitative assumptions can be made about the layer thickness. In this case, the more pronounced increase in resistance for 1 M LiPF ₆ indicated a thicker passivation layer than for electrolytes with 1 M LiFEES in TEG-DME, which adversely affects the electrochemical behavior of the cell.

These results show that lithium-air cells fabricated with lithium metal as counter electrode and carbon gas diffusion electrode had lower total resistance when using electrolytes with 1 M LiFEES in TEG-DME than lithium air. Cells with 1 M LiPF ₆ .

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• Method of Read et al., The Journal of Electrochemical Society, Vol. 149, page A1190 and Vol. 150, page A1351 [0048]

Claims (10)

Metal-air or metal-oxygen energy storage comprising a metal 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate salt. Energy store according to claim 1, characterized in that the energy store is selected from the group comprising lithium, sodium, magnesium, aluminum, zinc and / or iron-air or oxygen storage batteries or batteries. Energy store according to claim 1 or 2, characterized in that the energy store is a lithium-air or lithium-oxygen battery or battery. Energy store according to one of the preceding claims, characterized in that the metal salt is selected from the group comprising lithium, sodium, magnesium, aluminum, zinc and / or iron 2-pentafluoroethoxy-1,1,2,2- tetrafluoroethanesulfonate, especially lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate. Energy store according to one of the preceding claims, characterized in that the metal salt, in particular lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate, in an electrolyte comprising a solvent selected from the group comprising glycol dialkyl ethers and their oligomers and polymers , Amides, sulfoxides, nitriles, phosphonates, phosphate ethers, fluorinated ethers, substituted 2-oxazolidinones, ionic liquids and / or mixtures thereof, especially in tetraethylene glycol dimethyl ether. Electrolyte for a metal-air or metal-oxygen energy store comprising a metal 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate salt, in particular lithium 2-pentafluoroethoxy-1,1,2,2- tetrafluoroethanesulfonate, as an electrolyte salt and a solvent selected from the group comprising glycol dialkyl ethers of the general formula R ¹ -O- (CH ₂ CH ₂ -O) _n -R ² where R ¹ and R ² are each CH ₃ , C independently ₂ H ₅ , C ₃ H ₇ or C ₄ H ₉ , and n = 2 to 50, dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, 2-tolunitrile, benzonitrile, tris (2,2,2-trifluoroethyl) phosphate , Dimethyl sulfoxide and / or mixtures thereof. Electrolyte according to claim 6, characterized in that the solvent is tetraethylene glycol dimethyl ether. Electrolyte according to claim 6 or 7, characterized in that the concentration of the metal salt, in particular lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate, in the electrolyte in the range of ≥ 0.2 M to ≤ 1.6 M, preferably in the range of ≥ 0.6 M to ≦ 1.4 M, preferably in the range of ≥ 0.8 M to ≦ 1.2 M. Use of a metal 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate salt, in particular of lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate, in metal-air or metal Oxygen energy storage devices, in particular in lithium-air or lithium-oxygen storage batteries or batteries. Use according to claim 9, characterized in that the metal salt, in particular lithium 2-pentafluoroethoxy-1,1,2,2-tetrafluoroethanesulfonate, in an electrolyte comprising a solvent selected from the group comprising glycol dialkyl ethers of the general formula R ¹ -O - (CH ₂ CH ₂ -O) _n -R ² where R ¹ and R ² are each independently CH ₃ , C ₂ H ₅ , C ₃ H ₇ or C ₄ H ₉ and n = 2 to 50, dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, 2-tolunitrile, benzonitrile, tris (2,2,2-trifluoroethyl) phosphate, dimethyl sulfoxide and / or mixtures thereof, in particular in tetraethylene glycol dimethyl ether (TEG-DME) is used.

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